Research On Tunable Terahertz Devices Based On Graphene Metamaterial

The terahertz wave located between microwave and far-infrared light has unique performance compared to the microwave and millimeter wave band,which makes terahertz technology develop very fast exhibiting potential applications in different fields such as security,biomedical imaging,communications etc.Graphene,a two-dimensional layer of carbon atoms arranged in honeycomb pattern,has attracted remarkable attention in plasmonics due to its unique properties such as optical characteristics.It is found that graphene conductivity can be dynamically controlled in the terahertz frequency by changing the Fermi energy level based on chemical doping or gate voltage.Therefore,the tunability of optical devices can be achieved by using graphene.Many tunable devices based on graphene plasmon have been investigated such as absorber,sensors,switchers and modulators.In this paper,two novel terahertz-device models are proposed,and the two terahertz devices are simulated and analyzed.We proposed a tunable dual-band terahertz absorber based on graphene metamaterial,a bandwidth and center-frequency-tunable ultra-wideband terahertz absorber based on graphene,and another is a tunable dual Fano-resonance terahertz sensor based on graphene metamaterial.The major work in the thesis is summarized as follows:1.Firstly,the research status and application of terahertz technology are briefly introduced,which lays a theoretical foundation for the design,analysis and simulation of related devices based on terahertz.At the same time,the research status,application and several common preparation methods of graphene are also reviewed.2.The excellent properties of graphene are introduced.The parametric properties of graphene,including its conductivity and permittivity,are analyzed.The tunability of graphene is discussed,which lays the theoretical foundation for the design of tunable devices based on graphene.Finally,the theory of graphene surface plasmon polaritons is discussed.3.A tunable dual-band polarization-independent graphene-based metamaterial terahertz absorber is designed.Simulation results indicate that either of the absorber's two absorption bands can be tuned over a broad frequency range while the other absorption band remains unchanged.In addition,the absorber is fairly insensitive to the polarizations and incidence angles of incident wave.Meanwhile,a bandwidth and center-frequency-tunable graphene-based metamaterial terahertz broadband absorber is also designed.This absorber has not only wide absorption band but also tunable center frequency and bandwidth by changing the conductivity of graphene through external electric bias.Simulation results indicate that both the absorber's center frequency and bandwidth can be tuned over a broad frequency range.4.A dual Fano resonant terahertz sensor is proposed,which consists of two graphene square patches with different Fermi levels and an annular gold ring.Since it requires two different applied voltages,a two-layer design is beneficial for the implementation of experiment.The incidence of electromagnetic wave excites the surface plasmon polaritons?SPPs?of gold ring and the square graphene patches,respectively.Through a careful structural design one can overlap the resonances of the gold ring and the graphene square patches to excite Fano resonances through the coupling between the modes excited in the gold rings and the graphene square patches.Our simulation shows a sensitivity and figure of merit as high as 4.6329THz/RIU and 15.7047 RIU^-1,respectively.In addition,the Fermi levels of two graphene square patches can be tuned separately;therefore,the actively tunable sensing range of the sensor becomes wider.